I personally would stay with the person until other help came. I'm never so much "on a mission" when I go hiking that I have to stay on course no matter what, but that's me. I'm just not a serious hiker/camper, just a weekend warrior. It would take me awhile to get over it if I found out later that conditions changed and the injured person didn't make it.
Obviously it would depend on the circumstances, but that's pretty much my view, too. If it was a well marked trail and I knew with 99.5% certainty I could get to help and thought my companion would be okay til then, I'd go. But if we were lost in the first place I think it's safer to stay together. I might not be able to find my way back to him or her, plus there's the risk of there now being two lost people, lost in different locations. That would be twice the trouble for SAR.
If my companion was seriously hurt I'd be even more reluctant to leave, again, depending on the circumstances. Then again, maybe the only way your friend could survive would be to get evac'ed as fast as possible. Tough call.
As to the idea of making sure somebody can rescue me or her, I'm not totally understanding the concept. Please explain. How would you be able to know that? I'm thinking it would have to depend on the situation. If I feel a situation is serious, or about to become serious, then I would issue the international S-O-S signal using my whistle.
You never know who might be in the area. I'd probably try my whistle to see if anyone was around to respond. I can see that signaling on the move might be bad, but on well traveled trails it might be worth trying.